Many schemes have been proposed to support TCP traffic in a Differentiated Services network. We present in this paper an analytical model to study the performance of these schemes. The model is based on a Markovian fluid approach. We provide first a general version of the model, then we specify it to the different proposed schemes. For each scheme, we calculate the throughput achieved by a TCP connection. We compare then their service differentiation capacity under different subscription levels, different reservations, and different round-trip times.